1. Field of the invention
The present invention concerns a transmission system for the transmit part of a terminal associated with a digital mobile radio installation.
2. Description of the Prior Art
In a conventional digital mobile radio installation, radio coverage is currently organized in the following manner:
The coverage area is divided into a mosaic of hexagonal cells whose radius is between 1 km and 35 km, depending on the nature of the environment. It is in the order of 35 km in open country whereas it is in the order of 1 km to 3 km in urban areas.
A base transceiver station (BTS) is located at the center of each hexagonal cell in the coverage area and provides the radio link to all the mobile terminals within the perimeter of this cell. It therefore comprises all the radio frequency equipment needed to provide the coverage for a cell.
Each mobile terminal is a transceiver and has a microprocessor-based digital control and digital signal processing part and a radio frequency transceiver part.
The invention is concerned with the radio frequency amplifier system of the transmit part of a mobile terminal of this type.
The appended FIG. 1 outlines the constitution of a transmission system for a mobile terminal transmitter which is part of a prior art digital mobile radio installation.
The digital signal, made up of streams of bits, present at the input 1 of the transmit system is first phase modulated by a conventional "GMSK" modulator 2.
The modulator 2 supplies phase-modulated analog signals to two conventional output paths 3 and 4 in phase quadrature called by convention the "I" path and the "Q" path.
These are baseband signals. To be transmitted at radio frequency by the transmit antenna 4 the signals modulate a carrier, in a mixer unit 5, and are amplified in an amplifier system 6.
The mixer unit 5 conventionally comprises a local oscillator 7 whose output signals at a fixed frequency fo are fed to a first input of a simple mixer 8 which receives the Q path signals on its other input and, via a 90.degree. phase-shifter 9, to a first input of another simple mixer 10 which receives the I path signals on its other input.
The signals obtained at the outputs 11 and 12 of the simple mixers 10 and 8 are then combined on a common line 13. After conventional filtering (not shown) they are then passed to the amplifier system 6.
The rms amplitude of the signals at the mixer input on the two paths I and Q in phase quadrature is typically in the order of 100 mV.
The amplifier system 6 comprises in sequence a preamplifier 14, a variable attenuator 15 and a power amplifier 16.
The preamplifier 14, whose gain is typically in the order of 17 dB, for example, matches the output level of the mixer 5 to the input level of the power amplifier 16.
The variable attenuator 15 enables fine adjustment of the system 6 according to the tolerances of its components.
For a mobile terminal of this type, where the cost must be kept fairly low, the power amplifier 16 is a commercially available hybrid amplifier, of which there is a relatively restricted choice. For an input level of 0 dBm it typically supplies an output level between 13 dBm and 39 dBm.
The signal level at the input 13 of the preamplifier 14 is typically in the order of -15 dBm and that at the input 17 of the attenuator in the order of +2 dBm.
This prior art device gives rise to two manufacturing problems if the specifications for this kind of mobile radio installation are to be complied with, a first problem relating to conforming to the spectrum and being associated with signal modulation and a second problem concerning residual spurious amplitude modulation affecting the signal at the transmit antenna.
The specifications require all mobile telephone manufacturers to conform as closely as possible to a clearly defined theoretical spectrum. This theoretical spectrum includes clearly specified nominal rejections directly related to the modulation process itself.
The signals are conventionally transmitted in packets or "bursts" of digital information and the specifications state that for each burst the power must be transmitted in accordance with a clearly defined template. For example, it must be transmitted at between +1 dB and -1 dB relative to the mean value of the transmitted power, which in the final analysis characterizes the maximal amplitude modulation of the signal tolerated by the system.
Prior art devices have problems in conforming to these specifications.
A first problem concerns the inherent characteristics of the mixer employed, such as the mixer 5 in FIG. 1.
The conventional way to test the performance of a mixer of this kind is to use an auxiliary signal generator to inject into it a sinusoidal signal whose frequency fm represents one quarter of the required binary bit rate. The conventional binary bit rate is 270 kHz, so that the frequency fm is 67.7 kHz.
The signal at frequency fm is simultaneously injected with a relative phase-shift of 90.degree. into the I path 3 and into the Q path 4.
Ideally, there should be at the output 13 of the mixer 5 simply a pure component at the required frequency fo-fm.
Unfortunately, spurious frequencies are observed in practice, including the frequency fo of the local oscillator 7, constituting a local oscillator frequency residue, a spurious residue of the image frequency at fo+fm, distortion components and a third order intermodulation component 3.
It is these spurious frequency components which are responsible for the residual amplitude modulation of the modulated signal. Another problem is the non-linearity of the power amplifier 16 at the end of the transmit system: it is typically necessary to use power modules which operate in class AB, and the linearity of these power modules is far from optimal.
Because of the intrinsic characteristics of the modulation, on the output side of the mixer 5 the spectrum is inevitably asymmetrical. Downstream of the mixer it is therefore necessary to allow for amplifier stages which have an asymmetry which is the converse of that of this spectrum. The problem here is further degradation of performance in terms of amplitude modulation and spectrum (associated with modulation).